1. Field of the Invention
The present invention generally relates to a photographic rangefinder utilizing the principle of triangulation, and more particularly, to a position adjusting mechanism for a detector used in the rangefinder for automatic focusing purpose.
2. Description of the Related Art
FIG. 1 shows the principle of distance measurement based on triangulation used in a rangefinding system in, for example, a photographic camera.
Referring to FIG. 1, a beam of light LB emitted from a light emitting device 100 such as an LED is projected through a light projecting (condenser) lens 101 onto an area Ra of a subject S at a predetermined distance A or B from the lens 101, wherein the area Ra of the subject S has a width Rz and a height Ry, and Po represents the center of the area Ra. A beam of light LB so projected is reflected irregularly by the area Ra of the subject S, and the reflected light passes through a light receiving lens 112 onto a photodetector 111 such as a phototransistor so as to form an image of the subject S thereon.
In FIG. 1, Pa denotes an incident position of the reflected light from the subject S on the photodetector 111, and P denotes the center of the photodetector 111. The deviation amount Az of the reflected light corresponding to the distance between the light detecting position Pa on the photodetector 111 and the center position P of the photodetector 111 is expressed by the following equation: ##EQU1##
wherein L is the base length (the length of a base line BL) between the light projecting lens 101 and the light receiving lens 112, which corresponds to the distance between the centers of the light projecting lens 101 and the light receiving lens 112,
f is the focal length of the light receiving lens 112, and
A is the distance between the light projecting lens 101 and the subject S.
As shown in FIG. 1, the direction of the optical axis of a beam of light LB emitted from the light emitting device 100 through the light projecting lens 101 is referred to as an X- axis direction, a vertical direction thereof is referred to as a Y- axis direction, and a horizontal direction thereof which is orthogonal to the X- and Y- axis directions and is parallel to the base line BL is referred to as a Z- axis direction. In the triangulation type rangefinder constructed as described above, the following three conditions must be satisfied in order to determine the distance to the subject S precisely.
(1) Condition 1: A beam of light LB emitted from the light emitting device 100 through the light projecting lens 101 agrees with a beam of light passing through the center of the light receiving lens 112 and incident on the light detecting position Pa on the photodetector 111, at the center Po of the area Ra in the Z- axis direction under the condition wherein the distance from the system to the subject S is given A.
(2) Condition 2: The predetermined position on the photodetector 111 agrees with the center of the light receiving lens 112 in the Y- axis direction, and the center of the light emitting device 100 agrees with the center of the light projecting lens 101 in the Y- axis direction.
(3) Condition 3: The size of the image of the light emitting device 100 formed on the subject S is appropriate, and the image thereof is in focus in the X-axis direction under the condition of the predetermined distance A.
If the aforementioned condition 1 is not satisfied, as is apparent from the equation (1), obtained information is erroneously shifted from a proper value, and the accurate distance measurement is impossible.
If the aforementioned condition 2 is not satisfied, the image of the subject S to be subsequently formed on the photodetector 111 will not be formed on a light receiving surface of the photodetector 111, resulting in that the light amount incident onto the photodetector 111 may decrease, and the signal to noise ratio of the output of the photodetector 111 may decrease. In this case, a wrong distance may be determined.
If the aforementioned condition 3 is not satisfied, the image of the light emitting device 100 formed on the surface of the photodetector 111 may be too large, and a problem similar to that of the condition 2 may occur. Furthermore, if the image of the light emitting device 100 formed on the photodetector 111 is out of focus, the luminance distribution of the image thereof varies, resulting in that the deviation amount Az shown in FIG. 1 varies according to the distance to the subject S. In this case, a wrong distance may be determined.
In order to satisfy the aforementioned three conditions, there is provided a position adjusting mechanism for adjusting the light emitting device 100 in the X-, Y- and Z- axis directions so as to adjust the position thereof into the predetermined position. One example of the adjusting mechanism will be described below with reference to FIGS. 2 to 4.
FIG. 2 shows a longitudinal section of the light emitting device 100, and FIG. 3 is a top plan view thereof.
As shown in FIG. 2, the light emitting device is in the form of a single projection module including a light projecting lens la facing towards the subject or scene and a light emitting element 1 arranged behind the lens 1a so as to emit rays of light towards the lens 1a. The light emitting element 1 is mounted on a holder 2, and is adjustably biased in one direction, indicated by an arrow D shown in FIG. 2, by a plate spring 5 having one end portion 5a fixed to the holder 2. Furthermore, an adjustment screw 6 is screwed into the holder 2 in a direction counter to the direction in which the light emitting element 1 is normally biased by the plate spring 5, i.e., counter to the direction D so that a hemispherical end portion 6a of the adjustment screw 6 can be held in contact with a peripheral face of the light emitting element 1. The adjustment screw 6 presses the light emitting element 1 downward, as viewed in FIG. 2, against the biasing force of the plate spring 5. Furthermore, the position of the light emitting element 1 is regulated in the Z- axis direction by the holder 2. Accordingly, when the adjustment screw 6 is rotated, the position of the light emitting element 1 can be adjusted in the Y- axis direction by/against the plate spring 5.
The holder 2 is housed in a body 3 of the rangefinder. A pair of guiding grooves 3a which are parallel to the X- axis direction are formed on the top and bottom portions of the inner surface of the body 3 of the rangefinder. On the other hand, a pair of hemispherical projections 2b are formed on the top and bottom portions of the outer surface of the holder 2, and the hemispherical projections 2b are engaged in the guiding grooves 3a, respectively. Thus, the hemispherical projections 2b of the holder 2 can be slidably moved along the guiding grooves 3a, i.e., the X- axis direction. Furthermore, a leg portion 7a of a first eccentric shaft 7 is inserted rotatably into the holder 2, and a head portion 7b thereof is engaged slidably in a generally elliptical groove 3c formed in the top portion of the body 3 of the rangefinder. The elliptical groove 3c is so formed that the major axis (longitudinal direction) thereof extends orthogonal to the guiding groove 3a, and parallel to the Z- axis direction. Furthermore, a leg portion 8a of a second eccentric shaft 8 is inserted rotatably into the holder 2. A head portion 8b of the second eccentric shaft 8 is engaged slidably in a generally elliptical groove 3d formed in the top portion of the body 3 of the rangefinder. The elliptical groove 3d is so formed that the major axis (longitudinal direction) thereof extends generally parallel to the guiding groove 3a. Furthermore, a light projecting lens 4 is mounted on a front end portion of the body 3 of the rangefinder. It is to be noted that, within the body 3 of the rangefinder, a photodetector and a light receiving lens for converging the reflected light from the subject onto the photodetector are mounted is a known manner, however, the details thereof are omitted herein.
Next, the manner, in which the position of the light emitting element 1 in the rangefinder is adjusted, will be described hereinafter.
First of all, when the first eccentric shaft 7 is rotated in order to satisfy the aforementioned condition 3, i.e., to make the size of the image of the light emitting element formed on the photodetector appropriate, the leg portion 7a thereof is displaced relative to the head portion 7b thereof, with the head portion 7b causing the holder 2 to displace in the X- axis direction while the hemispherical projection 2b is guided along the guiding groove 3a and the second eccentric shaft 8 is guided along the elliptical groove 3d. The light emitting element 1 is therefore displaced relative to the light projecting lens 4 incident to this displacement of the holder 2, resulting in that the width of a beam of light projected by the light emitting element 1 can be adjusted.
Next, in order to satisfy the aforementioned condition 2, the adjustment screw 6 is rotated. By this rotation of the adjustment screw 6, the hemispherical end portion 6a of the adjustment screw 6 is displaced either upwards or downwards as viewed in FIG. 2, along the Y- axis direction relative to the holder 2, depending on the direction of turn of the screw 6. When the hemispherical end portion 6a thereof is moved upward, the light emitting element 1 is moved by the biasing force of the plate spring 5. On the other hand, when the hemispherical end portion 6a thereof is moved downward, the light emitting element 1 is moved downward against the biasing force of the plate spring 5. Thus, the light emitting element 1 is moved either upwards or downwards, and is displaced relative to the optical axis of the light projecting lens 4. Thus, a beam of light projected from the light emitting element 1 can be adjusted by the above relative displacement between the light projecting lens 4 and the light emitting element 1, and therefore, the light emitting element 1 can be brought in alignment with the optical axis of the photodetector in the Y- axis direction.
Finally, in order to satisfy the aforementioned condition 1, the second eccentric shaft 8 is rotated. Then, the leg portion 8a of the second eccentric shaft 8 is displaced relative to the head portion 8b thereof, the holder 2 connected to the leg portion 8a is rotated around the hemispherical projection 2b. According to the rotation of the holder 2, the projecting direction of the light emitting element 1 is moved in the Z- axis direction. By this movement of the light emitting element 1 in a direction parallel to the direction of the base length, a line linking the position Pa specified on the photodetector 111 with the center of the light receiving lens 112 can be brought into alignment with the optical axis of the light emitting element 1 at the center Po of the subject S at a predetermined distance A from the light projecting lens 101.
Another fine adjustment mechanism shown in FIG. 4 is provided for performing the highly precise adjustment in the Z- axis direction, so that the line linking the position Pa on the photodetector 111 with the center of the light receiving lens 112 can agree with the optical axis of the light emitting element 1 at the center Po of the subject S. In the fine adjustment mechanism, this adjustment can be performed by rotating an adjustment plate lens or glass 113 for adjusting the base length (the length L of the base line BL), which is supported for rotation about the Y- axis direction and is disposed generally intermediate between the light receiving lens 112 and the photodetector 111. Then, the relationship between the rotation angle .theta. of the adjustment plate glass 113 and the amount .DELTA.z of movement of the image of the light emitting device formed on the photodetector 111 is expressed by the following equation (2): ##EQU2##
wherein d is a thickness of the adjustment plate glass 113, and
n is a refractive index of the adjustment plate glass 113.
Under the condition of d=0.8 mm, n=1.4836, and .theta.=1.degree., a resolution .DELTA.z of 4.6 .mu.m is obtained.
In the conventional rangefinder described above, since the fine adjustment mechanism for the fine adjustment in the Z- axis direction is used in addition to the respective adjustment mechanisms for the adjustment in the X-, Y- and Z- axis directions, there are such disadvantages that it takes a long time to adjust the position of the light emitting element 1, and it is costly because it is comprised of a number of parts, etc..
In a compact camera, it is necessary to decrease the base length L and the focal length f of the light receiving lens 112 in the aforementioned equation (1) in order to make the camera compact. However, when the base length L and the focal length f are decreased for a given distance A, the deviation amount Az of the image formed on the photodetector 111 decreases, and therefore, it is necessary to highly precisely adjust respective parts of the rangefinder in order to satisfy all of the aforementioned three conditions.